The present invention relates to a permanent magnet motor and, more particularly, to a permanent magnet motor consisting of a rotor having twelve permanent magnets and a stator having nine magnetic poles.
Generally, permanent magnet motors have been used widely as control motors including servomotors because of their high efficiency and controllability.
FIG. 8 shows a conventional permanent magnet motor. This permanent magnet motor includes a rotor 12, which is formed by affixing C-shaped permanent magnets 10 in the radial direction onto the surface of a rotor yoke 11, and a stator consisting of a stator yoke 13 having a plurality of slots 15 arranged via a gap and a coil 17 wound on teeth 14. For this permanent magnet motor, the number of poles of the permanent magnet 10 is six, and the number of teeth is eighteen.
The torque of an AC servomotor etc. requiring highly accurate torque control must have small pulsation. Fluctuations in running torque caused by a change in magnetic flux distribution in a gap made by the positional relationship between the slot of stator and the permanent magnet at the time when the permanent magnet 10 is rotated, what is called cogging torque (torque in a state in which no current is caused to flow in the coil), occur, which exerts an adverse influence on uneven rotation and positioning accuracy.
To reduce this cogging torque, a method is used in which a C-shaped or D-shaped permanent magnet 20 whose center of outside diameter is shifted so that the end shape of the permanent magnet becomes thin.
In this method, there is proposed a technique in which the center of the arcuate shape of a rotor magnet portion facing towards the stator is shifted from the rotation center of the rotor, and the radius of the arcuate shape thereof is different from the radius of rotor. In FIG. 9, reference character 21 denotes the rotor yoke, R12 denotes the radius of curvature of the permanent magnet 20, R13 denotes the radius of the rotor yoke 21, and R15 denotes the radius of the rotor.
By using this method, the magnetic flux distribution in the permanent magnet end portion, which is a magnetic pole changeover portion in which a change in magnetic flux distribution is large, is made smooth, and hence the cogging torque can be reduced (refer to Japanese Patent Provisional Publication No. 2000-350393 as Patent Document 1).
However, for the above-described conventional permanent magnet motor, the optimal radius of the arcuate shape has conventionally been determined empirically or on a trial-and-error basis. Therefore, a study must be conducted for each motor.